This invention relates to a microwave receiving apparatus.
Recently, the development of a microwave transmitting and receiving system for transmitting and receiving high frequency signals such as the SHF band (e.g. 11.7-12.2 MHz) using a broadcasting satellite has been advanced. In the reception of such high frequency signals (hereinafter, referred to as microwaves), a remarkable amplification of the microwave is uneconomical. Generally, the microwave is first amplified by a low-noise and high-frequency amplifier, this microwave is converted into an intermediate frequency signal by a frequency converter using a mixer and a local oscilator whose oscillation frequency is lower than a reception signal, and thereafter this intermediate frequency signal is amplified. In this case, a image frequency signal is also converted into the intermediate frequency signal due to the frequency conversion process. Therefore, a band-pass filter is provided at the front stage of the frequency converter to suppress this image frequency signal. Such a band-pass filter is formed on a dielectric substrate as a microwave integrated circuit (MIC) by etching processes.
It is difficult to manufacture a large number of low-cost filters having low-loss and sharp characteristics taking account of a variation of filter elements in the etching processes. To solve this problem an image-rejection filter is also provided at the front stage of the high frequency amplifier. This filter is required to have a especially low-loss characteristic in order not to adversely affect the low-noise property of the high frequency amplifier. A waveguide filter is effective as a filter to meet such requirements.
FIG. 1 shows an outside appearance of a conventional microwave receiving apparatus using a waveguide filter. In the figure, a reference numeral 1 denotes a waveguide filter; 2 a receiving circuit board which constitutes a radio frequency amplifier, band-pass filter, frequency converter, and intermediate frequency amplifier; 3 a shield container for housing this receiving circuit board; and 4 an output terminal. This microwave receiving apparatus is generally installed outdoors as an outdoor unit together with an antenna (parabolic antenna).
The microwave receiving apparatus shown in FIG. 1 is difficult to be supported since its dimensions are large and its weight is large. To firmly support the apparatus, a heavy supporting member must be used, causing an increase in weight of the antenna. That is to say, this makes it difficult to install the antenna. As shown in FIG. 1, the attachment of the waveguide to the shield container results in an increase in length of the microwave receiving apparatus, so that the apparatus will be easily vibrated due to a wind or the like. When the receiving apparatus vibrates, the location of an input section 5 of the waveguide filter 1 changes. Thus, the radio waves received by the antenna are not correctly transmitted to the input section 5 of the waveguide filter 1, causing a deterioration of the receiving state.
In addition, a conventional microwave receiving apparatus requires three-stage connections of a primary reflector of an antenna, waveguide filter and receiving circuit, so that it includes problems on the alignment, attachment, or the like.